Fix systemd CVE-2018-15687

Originally created by @intrigeri on #16100 (Redmine)

• https://security-tracker.debian.org/tracker/CVE-2018-15687
• https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=912007

Related issues

• Related to #16097 (closed)
• Related to #16184 (closed)
• Blocks tails/accounting#15506